Demodulation reference signal (dmrs) sequence design for device-to-device (d2d) discovery

ABSTRACT

Technology for performing device-to-device (D2D) discovery is disclosed. A user equipment (UE) can identify a D2D discovery resource that is M subframes in a time domain, wherein M is a positive integer greater than one. The UE can generate K demodulation reference signal (DMRS) sequences to be transmitted from the UE for each subframe in the D2D discovery resource, wherein K is a positive integer greater than two. The UE can apply a predetermined orthogonal cover code (OCC) to each DMRS sequence. The predetermined OCC can be selected based on a value of M and a value of K. The UE can transmit the K DMRS sequences for each of the M subframes of the D2D discovery resource.

RELATED APPLICATIONS

This application claims the benefit of and hereby incorporates byreference U.S. Provisional Patent Application Ser. No. 61/990,643, filedMay 8, 2014, with an attorney docket number P67685Z.

BACKGROUND

Wireless mobile communication technology uses various standards andprotocols to transmit data between a node (e.g., a transmission station)and a wireless device (e.g., a mobile device). Some wireless devicescommunicate using orthogonal frequency-division multiple access (OFDMA)in a downlink (DL) transmission and single carrier frequency divisionmultiple access (SC-FDMA) in an uplink (UL) transmission. Standards andprotocols that use orthogonal frequency-division multiplexing (OFDM) forsignal transmission include the third generation partnership project(3GPP) long term evolution (LTE), the Institute of Electrical andElectronics Engineers (IEEE) 802.16 standard (e.g., 802.16e, 802.16m),which is commonly known to industry groups as WiMAX (Worldwideinteroperability for Microwave Access), and the IEEE 802.11 standard,which is commonly known to industry groups as WiFi.

In 3GPP radio access network (RAN) LTE systems, the node can be acombination of Evolved Universal Terrestrial Radio Access Network(E-UTRAN) Node Bs (also commonly denoted as evolved Node Bs, enhancedNode Bs, eNodeBs, or eNBs) and Radio Network Controllers (RNCs), whichcommunicates with the wireless device, known as a user equipment (UE).The downlink (DL) transmission can be a communication from the node(e.g., eNodeB) to the wireless device (e.g., UE), and the uplink (UL)transmission can be a communication from the wireless device to thenode.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the disclosure will be apparent from thedetailed description which follows, taken in conjunction with theaccompanying drawings, which together illustrate, by way of example,features of the disclosure; and, wherein:

FIG. 1 illustrates a device to device (D2D) discovery zone within an LTEoperation zone in accordance with an example;

FIGS. 2A-2C depict a set of orthogonal cover codes (OCCs) for each valueof demodulation reference signal (DMRS) sequences within a singlesubframe in accordance with an example;

FIG. 3 illustrates discovery resource mapping in a device to device(D2D) discovery zone in accordance with an example;

FIGS. 4A-4C depict a set of orthogonal cover codes (OCCs) for each valueof subframes within a device to device (D2D) discovery resource inaccordance with an example;

FIG. 5 depicts functionality of a user equipment (UE) operable toperform device-to-device (D2D) discovery in accordance with an example;

FIG. 6 depicts functionality of a user equipment (UE) operable toperform device-to-device (D2D) discovery in accordance with an example;

FIG. 7 depicts a flow chart of a method for performing device-to-device(D2D) discovery in accordance with an example;

FIG. 8 depicts functionality of a user equipment (UE) operable toperform device-to-device (D2D) discovery in accordance with an example;and

FIG. 9 illustrates a diagram of a wireless device (e.g., UE) inaccordance with an example.

Reference will now be made to the exemplary embodiments illustrated, andspecific language will be used herein to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended.

DETAILED DESCRIPTION

Before the present invention is disclosed and described, it is to beunderstood that this invention is not limited to the particularstructures, process steps, or materials disclosed herein, but isextended to equivalents thereof as would be recognized by thoseordinarily skilled in the relevant arts. It should also be understoodthat terminology employed herein is used for the purpose of describingparticular examples only and is not intended to be limiting. The samereference numerals in different drawings represent the same element.Numbers provided in flow charts and processes are provided for clarityin illustrating steps and operations and do not necessarily indicate aparticular order or sequence.

Example Embodiments

An initial overview of technology embodiments is provided below and thenspecific technology embodiments are described in further detail later.This initial summary is intended to aid readers in understanding thetechnology more quickly but is not intended to identify key features oressential features of the technology nor is it intended to limit thescope of the claimed subject matter.

A technology is described for a novel demodulation reference signal(DMRS) sequence design for device to device (D2D) discovery. When D2Ddiscovery is performed by a user equipment (UE), the UE can send a DMRSsequence using a discovery resource. The DMRS sequence can be selectedfrom a pool of DMRS sequences. When the discovery resource spansmultiple subframes, the UE can select a novel orthogonal code cover(OCC) to be applied to the DMRS sequences. When additional DMRSsequences are present within the discovery resource of a singlesubframe, the UE can select a novel OCC to be applied to the DMRSsequences. The application of the novel OCC can result in an increasedcapacity of the DMRS sequences. In one configuration, the DMRS sequencedesign can enable the UE to perform repeated transmissions of a D2Ddiscovery signal within a discovery period. The UE can randomly select afirst DMRS sequence to be transmitted using a first discovery resource,and then subsequent DMRS sequences that are associated with the firstDMRS sequence can be transmitted using subsequent discovery resourcesthat are associated with the first discovery resource, thereby reducinga processing power at the UE. In one configuration, the DMRS sequencedesign can be applicable to both Type 1 D2D discovery and Type 2 D2Ddiscovery. In addition, the UE can utilize only a subset of the pool ofDMRS sequences in order to reduce DMRS blind detection complexity, andthereby, UE power consumption.

Device to device (D2D) communication for 3GPP LTE networks, such as anEvolved Universal Terrestrial Radio Access network (E-UTRAN), is beingstandardized in 3GPP LTE Release 12. A D2D communication is a directcommunication between two devices, such as two user equipments (UEs).The two devices (e.g., LTE-based devices) can communicate directly withone another when the two devices are in close proximity, but such D2Dcommunications do not use the cellular network infrastructure. D2Ddiscovery is generally the first step performed at the UE to enable aD2D service. One particular application for D2D communications isrelated to public safety services. Furthermore, D2D communication canallow direct communication from one UE to one or more target orreceiving UEs, thus enabling group communication. Examples describedherein can refer to transmission to a target or receiving UE, but itshould be understood that this could also be a transmission to a groupof target or receiving UEs.

D2D can allow a direct link between two UEs that are using the cellularspectrum. As a result, media or other data can be transferred from onedevice to another device over short distances and using a directconnection. By using D2D data communications, the data can becommunicated directly without being relayed to the cellular network,thereby avoiding problems with lack of or poor network coverage or withoverloading the network. The cellular infrastructure, if present canassist with other issues, such as peer discovery, synchronization, andthe provision of identity and security information.

The use of D2D communication can provide several benefits to users. Forexample, the devices can be remote from cellular infrastructure. D2D canallow devices to communicate locally, even when the cellular network hasfailed (e.g., during a disaster) because D2D communication does not relyon the network infrastructure. By using licensed spectrum, thefrequencies used to perform the D2D communications are less subject tointerference. In addition, if the two devices are in close proximity,then reduced transmission power levels are used, thereby saving power atthe devices.

D2D communication features can be referred to as ProSe (ProximityServices) Direct Commination in the 3GPP LTE standard. D2Dcommunications are primarily targeted for public safety use cases, butcan be used for other applications as well. The D2D feature enables thedirect communication between UEs over the cellular radio spectrum, butwithout the data being carried by the cellular network infrastructure.D2D communication can occur when the UE is outside of the coverage ofthe cellular network, or alternatively, when the UE in within coverageof the cellular network. Within the access stratum of the UE, the D2Ddata can be carried by a D2D radio bearer.

In one example, a UE can transmit a D2D discovery message in order toperform D2D discovery. A physical uplink shared channel (PUSCH) can beused to transmit the D2D discovery message. The D2D discovery messagecan include a demodulation reference signal (DMRS) sequence. Inaddition, D2D discovery can utilize a cyclic redundancy code (CRC)between 16 and 24 bits, channel encoding (e.g., turbo or tail-bitingconvolutional codes), rate matching for bit size matching and thegeneration of multiple transmissions, and scrambling for interferencerandomization. The UE can transmit the D2D discovery message during adiscovery period. For each discovery period, the UE can transmit on arandomly selected discovery resource. When the UE is within a coveragearea of an evolved node B (eNB), the discovery period and the amount ofdiscovery resources can be configured by the eNB. In one example, thediscovery resource can have a duration of at least one millisecond (ms).The duration can be selected based on a size of a media access control(MAC) protocol data unit (PDU), in which case the duration can be amultiple of 1 ms and include consecutive D2D subframes. The discoveryresource can be used for a single transmission of a given discovery MACPDU by the UE.

When the UE is within network coverage, the eNB can periodicallyallocate certain discovery resources in the form of D2D discoveryregions for the UE. The UE can use these discovery resources in order totransmit discovery information. The discovery information can includeone or more DMRS sequences, which can be used to perform channelestimation, and timing offset and frequency offset estimation for uplinktransmissions. The discovery information can be in the form of adiscovery packet with payload information or a discovery packet precededby a discovery preamble. One or more resource blocks (RB) can be usedfor a discovery packet transmission during D2D discovery, which isdenoted as L_(RB) ^(D2D), depending on a payload size and overalldiscovery performance requirements.

FIG. 1 illustrates an exemplary device to device (D2D) discovery zonewithin an LTE operation zone. The LTE operation zone can be composed ofperiodic D2D discovery zones (DZ), wherein each DZ includes a definednumber of resource blocks (RBs) in a frequency domain and a definednumber of subframes in a time domain. As shown in FIG. 1, N_(RB) ^(D2D),n_(RB) ^(start), N_(SF) ^(D2D) and n_(SF) ^(start) are denoted as anumber of allocated RBs, a starting RB index, a number of subframes, anda starting subframe index of each discovery zone, respectively.

The UE can receive information regarding a partitioning of the LTEoperation zone (or D2D discovery regions) via semi-statically signalingfrom the eNB. For example, the eNB can use radio resource control (RRC)signaling to communicate the information to the UE. In particular, theeNB can send the information via a system information block (SIB) whenthe UE is within a coverage area of the eNB. When the UE has partialnetwork coverage, information regarding the configuration of thediscovery resources (i.e., the partitioning of the LTE operation zone)can be forwarded by one or a plurality of UEs that are within networkcoverage to the UE with partial network coverage. When the UE is out ofnetwork coverage, the discovery zone can be predefined or broadcasted bya centralized D2D device or be associated with and signaled by anindependent synchronization source, with the configuration furtherforwarded by other dependent/gateway synchronization sources.

In one configuration, a legacy PUSCH structure with a DMRS sequence canbe used for a D2D discovery message transmission. In previoustechniques, multiple mutually orthogonal reference signals can begenerated by employing different cyclic shifts of Zadoff-Chu sequenceand applying orthogonal cover codes to two reference-signaltransmissions within a subframe. More specifically, length-2 orthogonalcover codes [1 1] and [1 −1] can be utilized.

In Type 1 D2D discovery, the UE can perform contention based D2Ddiscovery or D2D discovery with UE-autonomous selection of discoveryresources. In other words, the UE can select the discovery resourcesused for transmitting D2D discovery messages (e.g., a discovery packet),as opposed to the eNB selecting the discovery resources. The UE can alsobe referred to as a ProSe enabled device. The UE can randomly select theDMRS sequence when transmitting the discovery packet. The DMRS sequencecan be used to perform channel estimation, and timing offset andfrequency offset estimation for uplink transmissions. Therefore, theDMRS sequence can enable D2D discovery to be performed by the UE. TheDMRS sequence can include a base sequence, which can be common with aplurality of other UEs or a function of the cell ID of the serving cellfor in-coverage UEs. In addition, the DMRS sequence can include a randomchoice of the cyclic shift and/or the orthogonal cover code (OCC) index.The discovering UEs can perform detection of discovery preambles orpacket detection in order to detect whether the discovery packet ispresent in the discovery resource. In addition, the discovering UEs canperform DM-RS identification in order to ensure appropriate channelestimation and timing/frequency offset compensation.

In legacy LTE solutions, 24 unique DMRS configurations are available.The 24 unique DMRS configurations are derived from 12 cyclic shifts and2 OCC indices. However, only a subset of the 24 DMRS sequences may beconfigured for D2D UEs in order to maintain sufficient orthogonalitybetween cyclic shifted versions of the same DM-RS base sequence. Inaddition, using the subset of the 24 DMRS sequences can ensurerobustness against delay spread introduced by practical channels, and toreduce the DMRS blind detection complexity. For instance, D2D UE canselect only one of the DMRS sequences for discovery packet transmissionfrom a subset of DMRS sequences with n_(cs)ε{0,4,8} and n_(oc)ε{0,1},where n_(cs) is the cyclic shift index and n_(oc) is the orthogonalcover code index. In this example, the D2D UE can select from a subsetconsisting of six DMRS sequences.

In one example, the configuration of the subset of DMRS sequences can bepre-configured or semi-statically signaled to the UE by the eNB usingRRC signaling, e.g., via system information blocks (SIBs) when the UE iswithin network coverage. For a partial network coverage scenario, theconfiguration can be forwarded by one or a plurality of UEs that arein-coverage to the UEs that are outside the network coverage area. Foran out-of-network coverage scenario, the configuration can be predefinedor broadcasted by a centralized D2D device. Alternatively, theconfiguration can be associated with and signaled by an independentsynchronization source, with the configuration further forwarded byother dependent/gateway synchronization sources.

In one configuration, whether the D2D discovery is for public safety(PS) or non-PS can influence a size of the D2D discovery messagetransmitted from the UE. In other words, the message sizes for these twotypes of D2D discovery can be different. To support D2D discovery withdifferent message sizes, the DMRS sequence can be used to carry one ormore information bits to signal a particular payload size. For example,one subset of DMRS sequences with n_(cs)ε{0,4,8} and n_(oc)ε{0} can beused to indicate a message size of X bits, while another subset of DMRSsequences with n_(cs)ε{0,4,8} and n_(oc)ε{1} can be used to indicate amessage size of Y bits.

The technology described herein relates to a novel DMRS sequence designfor D2D discovery. The DMRS sequence design can account for threedifferent scenarios: (1) a first scenario is for when a D2D discoveryresource is one subframe and a number of DMRS sequences (or symbols)within the one subframe is at least three; (2) a second scenario is forwhen the D2D discovery resource is at least two subframes; and (3) athird scenario is for when the D2D discovery resource is at least twosubframes and the number of DMRS sequences (or symbols) for eachsubframe is at least three.

With respect to the first scenario, a D2D discovery resource of a singlesubframe can be modified to include additional DMRS sequences (orsymbols). The additional DMRS sequences can improve a channel estimationperformance, as well as timing and frequency offset compensation. TheD2D discovery resource of the single subframe can be used by the UE tosend a D2D discovery message. In legacy solutions, only two DMRSsequences are within a single subframe. In the technology describedherein, the number of DMRS sequences within a single subframe can begreater than two. The number of DMRS sequences within the singlesubframe can be denoted as K, wherein K is a positive integer greaterthan two. In one example, a length-K Discrete Fourier Transform (DFT)based sequence or Walsh-Hadamard based sequence can be utilized for apredetermined orthogonal cover code (OCC) that is applied to each DMRSsequence. The predetermined OCC can be selected based on a value of K.The length-K DFT based sequence or the Walsh-Hadamard based sequence canbe utilized when the D2D discovery resource of the single subframe ismodified to include the additional DMRS sequences. The UE can apply thepredetermined OCC to each of the K DMRS sequences, and then transmit theK DMRS sequences using the D2D discovery resource of the singlesubframe. In one example, the UE can transmit the K DMRS sequences inthe D2D discovery message using a physical uplink shared channel(PUSCH).

FIG. 2A is a table depicting a set of orthogonal cover codes (OCCs) whenthe number (K) of DMRS sequences (or symbols) within a D2D discoveryresource of a single subframe is three (i.e., K=3). The set of OCCs canbe used for D2D discovery at the UE. The orthogonal sequences can bebased on a length-3 DFT code. The orthogonal sequences can berepresented as [w(o) . . . W(2)] when K=3. Each orthogonal sequence canbe associated with a particular sequence index n_(oc). For example, whenthe sequence index is 0, the orthogonal sequence is [1 1 1].

When the sequence index is 1, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 2\; {\pi/3}} & e^{j\; 4\; {\pi/3}}\end{bmatrix}.$

When the sequence index is 2, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 4\; {\pi/3}} & e^{j\; 2\; {\pi/3}}\end{bmatrix}$

Therefore, an appropriate orthogonal sequence can be applied to the DMRSsequence when K=3.

FIG. 2B is a table depicting a set of orthogonal cover codes (OCCs) whenthe number (K) of DMRS sequences (or symbols) within a D2D discoveryresource of a single subframe is four (i.e., K=4). The set of OCCs canbe used for D2D discovery at the UE. The orthogonal sequences can bebased on a length-4 Walsh-Hadamard code. The orthogonal sequences can berepresented as [w(o) . . . W(3)] when K=4. Each orthogonal sequence canbe associated with a particular sequence index n_(oc). For example, whenthe sequence index is 0, the orthogonal sequence is [+1 +1 +1 +1]. Whenthe sequence index is 1, the orthogonal sequence is [+1 −1 +1 −1]. Whenthe sequence index is 2, the orthogonal sequence is [+1 +1 −1 −1]. Whenthe sequence index is 3, the orthogonal sequence is [+1 −1 −1 +1].Therefore, an appropriate orthogonal sequence can be applied to the DMRSsequence when K=4.

FIG. 2C is a table depicting a set of orthogonal cover codes (OCCs) whenthe number (K) of DMRS sequences (or symbols) within a D2D discoveryresource of a single subframe is five (i.e., K=5). The set of OCCs canbe used for D2D discovery at the UE. The orthogonal sequences can bebased on a length-5 DFT code. The orthogonal sequences can berepresented as [w(o) . . . W(4)] when K=5. Each orthogonal sequence canbe associated with a particular sequence index n_(oc). For example, whenthe sequence index is 0, the orthogonal sequence is [1 1 1 1 1]. Whenthe sequence index is 1, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 2\; {\pi/5}} & e^{j\; 4\; {\pi/5}} & e^{j\; 6\; {\pi/5}} & e^{j\; 8\; {\pi/5}}\end{bmatrix}.$

When the sequence index is 2, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 4\; {\pi/5}} & e^{j\; 8\; {\pi/5}} & e^{j\; 2\; {\pi/5}} & e^{j\; 6\; {\pi/5}}\end{bmatrix}.$

When the sequence index is 3, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 6\; {\pi/5}} & e^{j\; 2\; {\pi/5}} & e^{j\; 8\; {\pi/5}} & e^{j\; 4\; {\pi/5}}\end{bmatrix}.$

When the sequence index is 4, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 8\; {\pi/5}} & e^{j\; 6\; {\pi/5}} & e^{j\; 4\; {\pi/5}} & e^{j\; 2\; {\pi/5}}\end{bmatrix}.$

Therefore, an appropriate orthogonal sequence can be applied to the DMRSsequence when K=5.

The design principles described above can be extended when K is greaterthan five DMRS sequences (or symbols), i.e., when K>5.

The UE can be configured to perform either Type 1 D2D discovery or Type2 D2D discovery. In Type 1 D2D discovery, the eNB can allocate a D2Ddiscovery resource pool to the UE via a system information block (SIB).The UE can randomly select the D2D discovery resource (e.g., a singlesubframe) from the D2D discovery resource pool and transmit the K DMRSsequences using the randomly selected D2D discovery resource. In Type 2D2D discovery, the UE can receive an indication of the D2D discoveryresource from the eNB via radio resource control (RRC) signaling. Inother words, in Type 2 D2D discovery, the UE can receive an allocationfor the D2D discovery resource from the eNB, and then transmit the KDMRS sequences using the allocated D2D discovery resource. The UE cantransmit the K DMRS sequences using the D2D discovery resource (e.g., asingle subframe) in order to perform channel estimation, timing offsetcompensation, and frequency offset compensation for D2D discovery. Aspreviously described, a novel orthogonal cover code can be applied toeach of the K DMRS sequences prior to transmission of the D2D discoverymessage with the DMRS sequence in the single subframe.

With respect to the second scenario, a D2D discovery resource ofmultiple subframes can be used to transmit the DMRS sequences (orsymbols) from the UE. The D2D discovery resource can span multiplesubframes and multiple physical resource blocks (PRBs). The D2Ddiscovery resource can be used by the UE to send a D2D discoverymessage. In legacy solutions, the D2D discovery resource would consistof only a single subframe. In the technology described herein, thenumber of subframes within the D2D discovery resource in a time domaincan be greater than one. The number of subframes within the D2Ddiscovery resource can be denoted as M, wherein M is a positive integergreater than one. Similar to the legacy solution, the number of DMRSsequences (or symbols) for each of the M subframes in this scenario istwo.

In one example, a length-2M DFT based sequence or Walsh-Hadamard basedsequence can be utilized for a predetermined orthogonal cover code (OCC)that is applied to each DMRS sequence. The predetermined OCC can beselected when multiple subframes are applied for discovery packettransmission. In other words, the predetermined OCC can be selectedbased on a value of M. The first length-2 sequence can applied for theDMRS symbols within the first subframe, the second length-2 sequence canapplied for the 2^(nd) subframe, and the M^(th) length-2 sequence canapplied for the M^(th) subframe.

In one configuration, the UE can generate two DMRS sequences to betransmitted for each subframe of the D2D discovery resource, wherein theD2D discovery resource can include multiple subframes. A predeterminedorthogonal cover code (OCC) can be applied to each DMRS sequence,wherein the predetermined OCC is selected based on a value of M. The UEcan transmit the two DMRS sequences for each of the M subframes of theD2D discovery resource using a physical uplink shared channel (PUSCH).The two DMRS sequences can be included in the D2D discovery message thatis transmitted from the UE. The UE can transmit the two DMRS sequencesfor each of the M subframes in order to perform channel estimation,timing offset compensation, and frequency offset compensation for D2Ddiscovery.

The UE can be configured to perform either Type 1 D2D discovery or Type2 D2D discovery. In Type 1 D2D discovery, the eNB can allocate a D2Ddiscovery resource pool to the UE via a system information block (SIB).The UE can randomly select the D2D discovery resource (e.g., multiplesubframes) from the D2D discovery resource pool and transmit two DMRSsequences using the multiple subframes of the randomly selected D2Ddiscovery resource. In Type 2 D2D discovery, the UE can receive anindication of the D2D discovery resource (e.g., multiple subframes) fromthe eNB via radio resource control (RRC) signaling. In other words, inType 2 D2D discovery, the UE can receive an allocation for the D2Ddiscovery resource from the eNB, and then transmit the two DMRSsequences using the allocated D2D discovery resource of multiplesubframes. As previously described, a novel orthogonal cover code can beapplied to each DMRS sequence prior to transmission of the D2D discoverymessage using the D2D discovery resource of the multiple subframes.

FIG. 3 illustrates an example of discovery resource mapping in a deviceto device (D2D) discovery zone. As previously explained, the discoveryresource can span multiple subframes and multiple physical resourceblocks (PRBs). In the example shown in FIG. 3, the discovery zone caninclude the discovery resource (i.e., the discovery resource can bemapped from the discovery zone). For the discovery resource, the numberof subframes in the time domain (denoted as M) is two and the number ofPRBs in the frequency domain (denoted as N) is two. In each subframe,the number of DMRS sequences can be two, as in a legacy PUSCHtransmission.

FIG. 4A is a table depicting a set of orthogonal cover codes (OCCs) whena D2D discovery resource spans two subframe (i.e., M=2). The set of OCCscan be used for D2D discovery at the UE. The orthogonal sequences can bebased on a length-4 Walsh-Hadamard based sequence. The orthogonalsequences can be represented as [w(o) L W(3)] when M=2. Each orthogonalsequence can be associated with a particular sequence index n_(oc). Forexample, when the sequence index is 0, the orthogonal sequence is [+1 +1+1 +1]. When the sequence index is 1, the orthogonal sequence is [+1 −1+1 −1]. When the sequence index is 2, the orthogonal sequence is [+1 +1−1 −1]. When the sequence index is 3, the orthogonal sequence is [+1 −1−1+1]. Therefore, an appropriate orthogonal sequence can be applied tothe DMRS sequence when M=2.

FIG. 4B is a table depicting a set of orthogonal cover codes (OCCs) whena D2D discovery resource spans three subframe (i.e., M=3). The set ofOCCs can be used for D2D discovery at the UE. The orthogonal sequencescan be based on a length-6 DFT code. The orthogonal sequences can berepresented as [w(o) L W(5)] when M=3. Each orthogonal sequence can beassociated with a particular sequence index n_(oc). For example, whenthe sequence index is 0, the orthogonal sequence is [1 1 1 1 1 1]. Whenthe sequence index is 1, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; {\pi \;/3}} & e^{j\; 2\; {\pi/3}} & {- 1} & e^{j\; 4\; {\pi/3}} & e^{j\; 5\; {\pi/3}}\end{bmatrix}.$

When the sequence index is 2, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 2\; {\pi \;/3}} & e^{j\; 4\; {\pi/3}} & 1 & e^{j\; 2{\pi/3}} & e^{j\; 4\; {\pi/3}}\end{bmatrix}.$

When the sequence index is 3, the orthogonal sequence is [1 −1 1 −1 1−1]. When the sequence index is 4, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 4\; {\pi \;/3}} & e^{j\; 2\; {\pi/3}} & 1 & e^{j\; 4{\pi/3}} & e^{j\; 2\; {\pi/3}}\end{bmatrix}.$

When the sequence index is 5, the orthogonal sequence is

$\begin{bmatrix}1 & e^{j\; 5\; {\pi \;/3}} & e^{j\; 4{\pi/3}} & {- 1} & e^{j\; 2\; {\pi/3}} & e^{j\mspace{11mu} {\pi/3}}\end{bmatrix}.$

Therefore, an appropriate orthogonal sequence can be applied to the DMRSsequence when M=3.

FIG. 4C is a table depicting a set of orthogonal cover codes (OCCs) whena D2D discovery resource spans four subframe (i.e., M=4). The set ofOCCs can be used for D2D discovery at the UE. The orthogonal sequencescan be based on a length-8 Walsh-Hadamard based sequence. The orthogonalsequences can be represented as [w(o) L W(7)] when M=4. Each orthogonalsequence can be associated with a particular sequence index n_(oc). Forexample, when the sequence index is 0, the orthogonal sequence is [+1 +1+1 +1 +1 +1 +1 +1]. When the sequence index is 1, the orthogonalsequence is [+1 −1 +1 −1 +1 −1 +1 −1]. When the sequence index is 2, theorthogonal sequence is [+1 +1 −1 −1 +1 +1 −1 −1]. When the sequenceindex is 3, the orthogonal sequence is [+1 −1 −1 +1 +1 −1 −1 +1]. Whenthe sequence index is 4, the orthogonal sequence is [+1 +1 +1 +1 −1 −1−1 −1]. When the sequence index is 5, the orthogonal sequence is [+1 −1+1 −1 −1 +1 −1 +1]. When the sequence index is 6, the orthogonalsequence is [+1 +1 −1 −1 −1 −1 +1 +1]. When the sequence index is 7, theorthogonal sequence is [+1 −1 −1 +1 −1 +1 +1 −1]. Therefore, anappropriate orthogonal sequence can be applied to the DMRS sequence whenM=4.

The design principles described above can be extended when M is greaterthan four subframes per D2D discovery resource i.e., when M>4.

With respect to the third scenario, a D2D discovery resource can spanmultiple subframes and additional DMRS sequences (or symbols) areincluded for each subframe of the D2D discovery resource. The number ofsubframes in a time domain within the D2D discovery resource can bedenoted as M, wherein M is a positive integer greater than one. Thenumber of DMRS sequences for each subframe within the D2D discoveryresource can be denoted as K, wherein K is a positive integer greaterthan two.

In one example, a length-MK DFT based sequence or Walsh-Hadamard basedsequence can be utilized for a predetermined orthogonal cover code (OCC)that is applied to each DMRS sequence. The predetermined OCC can beapplied when the D2D discovery resource spans multiple subframes andadditional DMRS symbols are included in each subframe. The firstlength-K sequence can be applied for the first DMRS sequence within thefirst subframe, the second length-K sequence can be applied for thesecond subframe, and the M^(th) length-K sequence can be applied for theM^(th) subframe.

In one configuration, the UE can generate K DMRS sequences to betransmitted for each subframe of the D2D discovery resource, wherein theD2D discovery resource can include multiple subframes. A predeterminedorthogonal cover code (OCC) can be applied to each DMRS sequence,wherein the predetermined OCC is selected based on a value of M and avalue of K. The UE can transmit the K DMRS sequences for each of the Msubframes of the D2D discovery resource using a physical uplink sharedchannel (PUSCH). The K DMRS sequences can be included in the D2Ddiscovery message that is transmitted from the UE. The UE can transmitthe K DMRS sequences for each of the M subframes in order to performchannel estimation, timing offset compensation, and frequency offsetcompensation for D2D discovery. In this scenario, the total number ofDMRS sequences in one discovery resource can be calculated as M×K,wherein M is the number of subframes for the discovery packettransmission and K is the number of DMRS symbols within one subframe.

When M=2 and K=3, a length-6 DFT based sequence is utilized for anorthogonal sequence, wherein the length-6 DFT based sequence is similarto as described above.

When M=2 and K=4, a length-8 Walsh-Hadamard based sequence is utilizedfor an orthogonal sequence, wherein the length-8 Walsh-Hadamard basedsequence is similar to as described above.

The design principles described above can be extended when M is greaterthan two subframes and/or K is greater than two DMRS symbols, i.e., whenM>2 and/or K>2.

In one configuration, the UE can perform a repeated transmission of theDMRS sequences for Type 1 D2D discovery. A novel DMRS sequence designfor repeated transmissions is described herein. The UE can repeatedlytransmit (e.g., either contiguously or non-contiguously) a givendiscovery MAC PDU within a discovery period. The eNB can allocate a D2Ddiscovery resource pool (or a set of discovery resources) for the UE.The resources in the D2D discovery resource pool can be used for therepeated transmissions of the discovery MAC PDU. The UE can perform arandom selection of a first resource from the D2D discovery pool. The UEcan use the first resource for transmitting a first discovery MAC PDU.The UE can subsequently use other resources for subsequent discovery MACPDU transmissions, wherein the other resources are deterministicallyassociated with the first resource. Alternatively, the UE can performrandom selection for each resource in a D2D discovery resource pool (asopposed to the random selection of only the first resource). The UE canbe configured to perform a defined number of repeated transmissions.

For repeated transmission, ProSe-enabled UEs can transmit multiplecopies of the discovery packets within one D2D discovery zone. Inparticular, each D2D discovery zone (DZ) can be divided into N sub-DZs,and the D2D UEs can transmit one discovery packet in each sub-DZ. Therecan be two scenarios with respect to repeated transmissions of thediscovery packets from the UE. In the first scenario, the UE canrandomly select the resource only for the first transmission, and theresources for subsequent transmissions are deterministically associatedwith the first resource. In other words, a frequency and time locationof the subsequent transmission is determined by the initialtransmission. The UE can randomly select the resource from a D2Ddiscovery resource pool that is allocated by the eNB. In addition, theUE can randomly choose one DMRS sequence for transmission within thisselected discovery resource. In other words, the UE can transmit theDMRS sequence using the discovery resource selected from the D2Ddiscovery resource pool. The UE can subsequently transmit another DMRSsequence that is related (or identical) to the initial DMRS sequence.Even though the subsequent transmission can be performed using anotherdiscovery resource that is selected from a separate D2D discoveryresource pool, the subsequent discovery resource can be derived from theinitial discovery resource. In the second scenario, the UE can randomlyselect the resources for all the transmissions. In other words, in thisscenario, the resources that are subsequently selected are notassociated with the resource that is initially selected. In addition,the UE can randomly select the DMRS sequence for each of the randomlyselected resources. In this scenario, the subsequent DMRS sequences arenot associated with the initial DMRS sequence.

With respect to the first scenario, there are several manners in whichthe UE can perform the DMRS sequence selection. In one example, the UEcan randomly select the DMRS sequences from a DMRS sequence pool for thefirst transmission. The DMRS sequence pool can be a subset of allavailable DMRS sequences and can be predefined or configured by thenetwork, or alternatively, the DMRS sequence pool can be a full set ofall available DMRS sequences. The UE can perform the first transmissionusing a randomly selected resource. The UE can randomly select theresource from a D2D discovery resource pool that is allocated by theeNB. For the subsequent transmission, the UE can choose an identicalDMRS sequence as the first transmission. Each subsequent transmission ofthe DMRS sequence can be performed using discovery resources that areselected from separate discovery resource pools allocated by the eNB.Therefore, the discovering UE can combine the correlation energy ofmultiple transmissions to improve DMRS detection performance.

In another example, the UE can randomly select the DMRS sequence from aDMRS sequence pool for the first transmission. The UE can generate asubsequent DMRS sequence by performing DMRS sequence hopping on theinitial DMRS sequence (i.e., for the first transmission). In otherwords, the subsequent DMRS sequence can be associated with the initialDMRS sequence after DMRS sequence hopping is performed, but the two DMRSsequences are not identical. A DMRS sequence hopping pattern can becell-specific or common across the network to allow for efficientdiscovery. In some examples, either base sequence hopping, or cyclicshift hopping or orthogonal cover code hopping or any combination of theabove options can be utilized for DMRS sequence hopping. For instance,for cyclic shift hopping, the UE can select a DMRS sequence for thefirst transmission to have a cyclic shift index n_(cs). In thesubsequent transmission, the UE can transmit a DMRS sequence with cyclicshift index of:

n_(cs)(k)=(n_(cs)+k·L)mod N_(cs), wherein k is a repeated transmissionindex, L is a hopping distance (which can be predefined or signaled by ahigher layer), and N_(cs) is the total number of cyclic shifts (e.g., 12cyclic shifts). In another example, the hopping pattern for the DMRSsequence can be the same as the hopping pattern applied for resourcehopping with respect to repeated transmissions.

In yet another example, the UE can randomly select the DMRS sequencesfor all of the transmissions. For this scheme, consistent collision ofDMRS sequence transmissions can be avoided. However, the detection gainfrom combining multiple received copies of the DMRS symbols cannot berealized.

With respect to the second scenario, the UE can randomly select theresources for all of the transmissions. The UE can randomly select theresources from a plurality of D2D discovery resource pools allocated bythe eNB. In this case, an initial resource used for transmitting thediscovery packet is not related to or associated with a subsequentresource used for transmitting a later discovery packet. In addition,the UE can randomly select the DMRS sequences for all of thetransmissions. Unlike the previous scenario, an initial DMRS sequenceselected for transmission is not identical to or associated with asubsequent DMRS sequence that is selected for transmission. The UE cantransmit a first randomly selected DMRS sequence using a first randomlyselected discovery resource, and then subsequently transmit a secondrandomly selected DMRS sequence using a second randomly selecteddiscovery resource, wherein the DMRS sequence and discovery resourcefrom the first transmission is not related to the DMRS sequence anddiscovery resource from the second transmission.

In one configuration, the UE can perform a repeated transmission of theDMRS sequences for Type 2 D2D discovery. A novel DMRS sequence designcan be applicable for Type 2 D2D discovery. Type 2 D2D discovery is aprocedure where resources for discovery signal transmissions areallocated on a per UE specific basis. In Type 2A, resources areallocated for each specific transmission instance of discovery signals.In Type 2B, resources are semi-persistently allocated for discoverysignal transmissions. Type 2 discovery can be controlled by the eNB andnot the UE. In contrast, Type 1 D2D discovery is a discovery procedurewhere resources for discovery signal transmissions are allocated on anon-UE specific basis. The resources can be for all UEs or a group ofUEs. Type 1 D2D is for contention based D2D discovery, such that the UErandomly chooses the D2D discovery resource for the transmission. Theutilization of DMRS base sequences for Type 2 discovery is subject tosimilar considerations as for Type 1 discovery. The DMRS sequences caneither be network-common or pre-configured or associated with asynchronization source identity.

Several options can be considered for DMRS sequence design for Type 2discovery. In the first option, a D2D transmitter (Tx) UE can randomlyselect a DMRS sequence with a defined cyclic shift (CS) and a definedorthogonal cover code (OCC). The D2D Tx UE can select the DMRS sequencefor transmission in each discovery period (for Type 2B discovery) or foreach transmission instance (for Type 2A discovery). In the secondoption, the D2D Tx UE can apply DMRS sequence hopping for thetransmission in each discovery period (for Type 2B discovery). Aninitial choice of the DMRS sequence can be randomly chosen by the UE orassigned by the eNB. In the third option, the D2D Tx UE can use the sameDMRS sequence for the transmission in each discovery period (for Type 2Bdiscovery). An initial choice of the DMRS sequence can be randomlychosen by the UE or assigned by the eNB. When repeated transmissions areconfigured or allowed for Type 2 discovery, similar options as comparedto Type 1 discovery can apply for DMRS sequence choices within adiscovery period.

In one configuration, for Type 1 discovery, a subset of DMRS sequencescan be configured for ProSe-enabled devices in order to reduce DMRSblind detection complexity at the UE. A reduction of DMRS blinddetection complexity can result in power consumption savings at the UE.To further improve the orthogonality and channel separation, especiallyin the presence of practical impairment, DMRS sequences with relativelylarge cyclic shift (CS) separation and appropriate orthogonal covercodes (OCCs) can be configured.

In one example, two OCCs with an identical CS may not be configured forthe DMRS sequence. For instance, OCC index 0 and 1 cannot be configuredtogether with CS index 0. This is primarily due to the fact that certainambiguity occurs between phase rotations of two DMRS symbols introducedby the OCC and large frequency offset. In this case, discovering UE maybe unable to differentiate the OCC or estimate frequency offsetcorrectly. As a result, the discovering UE may be unable to identify thecorrect DMRS sequences.

In another example, two OCCs with a CS offset can be configured for theDMRS sequence. As a non-limiting example, the configuration can be DMRSConfiguration I: CS and OCC index {n_(cs), n_(occ)}ε{{0,0}, {3,1},{6,0},{9,1}}. In this case, the total number of DMRS sequences is 4. Asanother non-limiting example, the configuration can be DMRSConfiguration II: CS and OCC index {n_(cs), n_(occ)} E {{0,0}, {2,1},{4,0},{6,1},{8,0},{10,1}}. In this case, the total number of DMRSsequence is 6. Although DMRS Configuration I can outperform DMRSConfiguration II in terms of DMRS blind detection performance, the DM-RScollision probability can be higher for DMRS Configuration I as comparedto DMRS Configuration II.

In yet another example, only a CS with a single OCC index can beconfigured for the DMRS sequence. Two non-limiting examples of suchconfigurations can be DMRS Configuration IA: CS and OCC index {n_(cs),n_(occ)}ε{{0,0}, {3,0}, {6,0},{9,0}} and DMRS Configuration IIA: CS andOCC index {n_(cs), n_(occ)}ε{{0,0}, {2,0}, {4,0},{6,0},{8,0},{10,0}}.

Another example provides functionality 500 of a user equipment (UE)comprising one or more processors configured to perform device-to-device(D2D) discovery, as shown in the flow chart in FIG. 5. The functionalitycan be implemented as a method or the functionality can be executed asinstructions on a machine, where the instructions are included on atleast one computer readable medium or one non-transitory machinereadable storage medium. The one or more processors can be configured toidentify a D2D discovery resource of a single subframe, as in block 510.The one or more processors can be configured to generate K demodulationreference signal (DMRS) sequences to be transmitted from the UE usingthe D2D discovery resource, wherein K is a positive integer greater thantwo, as in block 520. The one or more processors can be configured toapply an orthogonal cover code (OCC) to each DMRS sequence, wherein theOCC is randomly selected from a pool of OCCs, wherein the OCCs in thepool are predefined based on a value of K, as in block 530. The one ormore processors can be configured to transmit the K DMRS sequences usingthe D2D discovery resource of the single subframe, as in block 540.

In one example, the one or more processors are further configured totransmit the K DMRS sequences in a D2D discovery message using aphysical uplink shared channel (PUSCH). In another example, the one ormore processors are further configured to randomly select the D2Ddiscovery resource from a D2D discovery resource pool, wherein the D2Ddiscovery resource pool is allocated by an evolved node B (eNB) andindicated to the UE via a system information block (SIB). In yet anotherexample, the one or more processors are further configured to receive anindication of the D2D discovery resource from an evolved node B (eNB)via radio resource control (RRC) signaling.

In one example, the one or more processors are further configured totransmit the K DMRS sequences using the single subframe of the D2Ddiscovery resource. In another example, the UE is configured to performType 1 D2D discovery or Type 2 D2D discovery. In yet another example,the UE includes an antenna, a touch sensitive display screen, a speaker,a microphone, a graphics processor, an application processor, aninternal memory, or a non-volatile memory port.

Another example provides functionality 600 of a user equipment (UE)comprising one or more processors configured to perform device-to-device(D2D) discovery, as shown in the flow chart in FIG. 6. The functionalitycan be implemented as a method or the functionality can be executed asinstructions on a machine, where the instructions are included on atleast one computer readable medium or one non-transitory machinereadable storage medium. The one or more processors can be configured toidentify a D2D discovery resource that is M subframes in a time domain,wherein M is a positive integer greater than one, as in block 610. Theone or more processors can be configured to generate two demodulationreference signal (DMRS) sequences to be transmitted from the UE for eachsubframe in the D2D discovery resource, as in block 620. The one or moreprocessors can be configured to apply an orthogonal cover code (OCC) toeach DMRS sequence, wherein the OCC is randomly selected from a pool ofOCCs, wherein the OCCs in the pool are predefined based on a value of M,as in block 630. The one or more processors can be configured totransmit the two DMRS sequences for each of the M subframes of the D2Ddiscovery resource, as in block 640.

In one example, the one or more processors are further configured totransmit the two DMRS sequences in a D2D discovery message using aphysical uplink shared channel (PUSCH). In another example, the one ormore processors are further configured to randomly select the D2Ddiscovery resource from a D2D discovery resource pool allocated by anevolved node B (eNB). In yet another example, the one or more processorsare further configured to transmit the two DMRS sequences for each ofthe M subframes.

Another example provides a method 700 for performing device-to-device(D2D) discovery, as shown in the flow chart in FIG. 7. The method can beexecuted as instructions on a machine, where the instructions areincluded on at least one computer readable medium or one non-transitorymachine readable storage medium. The method can include the operation ofidentifying, at a user equipment (UE), a D2D discovery resource that isM subframes in a time domain, wherein M is a positive integer greaterthan one, as in block 710. The method can include the operation ofgenerating K demodulation reference signal (DMRS) sequences to betransmitted from the UE for each subframe in the D2D discovery resource,wherein K is a positive integer greater than two, as in block 720. Themethod can include the operation of applying an orthogonal cover code(OCC) to each DMRS sequence, wherein the OCC is randomly selected from apool of OCCs, wherein the OCCs in the pool are predefined based on avalue of M and a value of K, as in block 730. The method can include theoperation of transmitting the K DMRS sequences for each of the Msubframes of the D2D discovery resource from the UE, as in block 740.

In one example, the K DMRS sequences for each of the M subframes aretransmitted using a physical uplink shared channel (PUSCH). In anotherexample, Type 1 D2D discovery or Type 2 D2D discovery is performed atthe UE.

FIG. 8 depicts functionality of a user equipment (UE) 800 operable toperform device-to-device (D2D) discovery. The UE 800 can include aselection module 802 configured to select a first demodulation referencesignal (DMRS) sequence from a pool of DMRS sequences for D2D discovery.The selection module 802 can be configured to select a first D2Ddiscovery resource from a first D2D discovery resource pool allocated byan evolved node B. The UE 800 can include a communication module 804configured to transmit the first DMRS sequence from the UE 800 using thefirst D2D discovery resource selected from the D2D discovery resourcepool, wherein a second DMRS sequence is subsequently transmitted fromthe UE 800 using a second D2D discovery resource that is selected from asecond D2D discovery resource pool allocated by the eNB

In one example, the selection module 802 can be further configured to:randomly select the first DMRS sequence from the pool of DMRS sequences;and select the second D2D discovery resource based on the first D2Ddiscovery resource. In another example, the communication module 804 canbe further configured to transmit a DMRS sequence in each discoverysubzone within a configured discovery period. In yet another example,the second DMRS sequence is identical to the first DMRS sequence

In one example, the UE 800 can include a generation module 806configured to generate the second DMRS sequence by performing DMRSsequence hopping on the first DMRS sequence, wherein the DMRS sequencehopping utilizes at least one of: base sequence hopping, cyclic shifthopping, or orthogonal code cover hopping. In another example, theselection module 804 can be further configured to randomly select thefirst D2D discovery resource from the D2D discovery resource pool,wherein the second D2D discovery resource is deterministicallyassociated with the first D2D discovery resource. In yet anotherexample, the selection module 804 can be further configured to randomlyselect the first DMRS sequence and the second DMRS sequence atsubstantially a same time, wherein the second DMRS sequence isdistinguishable from the first DMRS sequence.

In one example, the selection module 804 can be further configured torandomly select the first D2D discovery resource and the second D2Ddiscovery resource at substantially a same time, wherein the second D2Ddiscovery resource is not associated with the first D2D discoveryresource. In another example, the UE 800 is configured to perform Type 1D2D discovery or Type 2 D2D discovery. In yet another example, the UE800 can include a discovery module 808 configured to perform the D2Ddiscovery using a subset of the pool of DMRS sequences, wherein eachDMRS sequence in the subset is associated with a configured number ofcyclic shift (CS) and a configured number of orthogonal cover codes(OCCs).

FIG. 9 provides an example illustration of the wireless device, such asa user equipment (UE), a mobile station (MS), a mobile wireless device,a mobile communication device, a tablet, a handset, or other type ofwireless device. The wireless device can include one or more antennasconfigured to communicate with a node or transmission station, such as abase station (BS), an evolved Node B (eNB), a baseband unit (BBU), aremote radio head (RRH), a remote radio equipment (RRE), a relay station(RS), a radio equipment (RE), a remote radio unit (RRU), a centralprocessing module (CPM), or other type of wireless wide area network(WWAN) access point. The wireless device can be configured tocommunicate using at least one wireless communication standard including3GPP LTE, WiMAX, High Speed Packet Access (HSPA), Bluetooth, and WiFi.The wireless device can communicate using separate antennas for eachwireless communication standard or shared antennas for multiple wirelesscommunication standards. The wireless device can communicate in awireless local area network (WLAN), a wireless personal area network(WPAN), and/or a WWAN.

FIG. 9 also provides an illustration of a microphone and one or morespeakers that can be used for audio input and output from the wirelessdevice. The display screen may be a liquid crystal display (LCD) screen,or other type of display screen such as an organic light emitting diode(OLED) display. The display screen can be configured as a touch screen.The touch screen may use capacitive, resistive, or another type of touchscreen technology. An application processor and a graphics processor canbe coupled to internal memory to provide processing and displaycapabilities. A non-volatile memory port can also be used to providedata input/output options to a user. The non-volatile memory port mayalso be used to expand the memory capabilities of the wireless device. Akeyboard may be integrated with the wireless device or wirelesslyconnected to the wireless device to provide additional user input. Avirtual keyboard may also be provided using the touch screen.

Various techniques, or certain aspects or portions thereof, may take theform of program code (i.e., instructions) embodied in tangible media,such as floppy diskettes, compact disc-read-only memory (CD-ROMs), harddrives, non-transitory computer readable storage medium, or any othermachine-readable storage medium wherein, when the program code is loadedinto and executed by a machine, such as a computer, the machine becomesan apparatus for practicing the various techniques. Circuitry caninclude hardware, firmware, program code, executable code, computerinstructions, and/or software. A non-transitory computer readablestorage medium can be a computer readable storage medium that does notinclude signal. In the case of program code execution on programmablecomputers, the computing device may include a processor, a storagemedium readable by the processor (including volatile and non-volatilememory and/or storage elements), at least one input device, and at leastone output device. The volatile and non-volatile memory and/or storageelements may be a random-access memory (RAM), erasable programmable readonly memory (EPROM), flash drive, optical drive, magnetic hard drive,solid state drive, or other medium for storing electronic data. The nodeand wireless device may also include a transceiver module (i.e.,transceiver), a counter module (i.e., counter), a processing module(i.e., processor), and/or a clock module (i.e., clock) or timer module(i.e., timer). One or more programs that may implement or utilize thevarious techniques described herein may use an application programminginterface (API), reusable controls, and the like. Such programs may beimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the program(s)may be implemented in assembly or machine language, if desired. In anycase, the language may be a compiled or interpreted language, andcombined with hardware implementations.

As used herein, the term processor can include general purposeprocessors, specialized processors such as VLSI, FPGAs, or other typesof specialized processors, as well as base band processors used intransceivers to send, receive, and process wireless communications.

It should be understood that many of the functional units described inthis specification have been labeled as modules, in order to moreparticularly emphasize their implementation independence. For example, amodule may be implemented as a hardware circuit comprising customvery-large-scale integration (VLSI) circuits or gate arrays,off-the-shelf semiconductors such as logic chips, transistors, or otherdiscrete components. A module may also be implemented in programmablehardware devices such as field programmable gate arrays, programmablearray logic, programmable logic devices or the like.

Modules may also be implemented in software for execution by varioustypes of processors. An identified module of executable code may, forinstance, comprise one or more physical or logical blocks of computerinstructions, which may, for instance, be organized as an object,procedure, or function. Nevertheless, the executables of an identifiedmodule need not be physically located together, but may comprisedisparate instructions stored in different locations which, when joinedlogically together, comprise the module and achieve the stated purposefor the module.

Indeed, a module of executable code may be a single instruction, or manyinstructions, and may even be distributed over several different codesegments, among different programs, and across several memory devices.Similarly, operational data may be identified and illustrated hereinwithin modules, and may be embodied in any suitable form and organizedwithin any suitable type of data structure. The operational data may becollected as a single data set, or may be distributed over differentlocations including over different storage devices, and may exist, atleast partially, merely as electronic signals on a system or network.The modules may be passive or active, including agents operable toperform desired functions.

Reference throughout this specification to “an example” or “exemplary”means that a particular feature, structure, or characteristic describedin connection with the example is included in at least one embodiment ofthe present invention. Thus, appearances of the phrases “in an example”or the word “exemplary” in various places throughout this specificationare not necessarily all referring to the same embodiment.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary. In addition, various embodiments and example of the presentinvention may be referred to herein along with alternatives for thevarious components thereof. It is understood that such embodiments,examples, and alternatives are not to be construed as defactoequivalents of one another, but are to be considered as separate andautonomous representations of the present invention.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided, such asexamples of layouts, distances, network examples, etc., to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that the invention can bepracticed without one or more of the specific details, or with othermethods, components, layouts, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by the claims setforth below.

What is claimed is:
 1. A user equipment (UE) operable to performdevice-to-device (D2D) discovery, the UE comprising one or moreprocessors configured to: identify a D2D discovery resource of a singlesubframe; generate K demodulation reference signal (DMRS) sequences tobe transmitted from the UE using the D2D discovery resource, wherein Kis a positive integer greater than two; apply an orthogonal cover code(OCC) to each DMRS sequence, wherein the OCC is randomly selected from apool of OCCs, wherein the OCCs in the pool are predefined based on avalue of K; and transmit the K DMRS sequences using the D2D discoveryresource of the single subframe.
 2. The UE of claim 1, wherein the oneor more processors are further configured to transmit the K DMRSsequences in a D2D discovery message using a physical uplink sharedchannel (PUSCH).
 3. The UE of claim 1, wherein the one or moreprocessors are further configured to randomly select the D2D discoveryresource from a D2D discovery resource pool, wherein the D2D discoveryresource pool is allocated by an evolved node B (eNB) and indicated tothe UE via a system information block (SIB).
 4. The UE of claim 1,wherein the one or more processors are further configured to receive anindication of the D2D discovery resource from an evolved node B (eNB)via radio resource control (RRC) signaling.
 5. The UE of claim 1,wherein the one or more processors are further configured to transmitthe K DMRS sequences using the single subframe of the D2D discoveryresource.
 6. The UE of claim 1, wherein the UE is configured to performType 1 D2D discovery or Type 2 D2D discovery.
 7. The UE of claim 1,wherein the UE includes an antenna, a touch sensitive display screen, aspeaker, a microphone, a graphics processor, an application processor,an internal memory, or a non-volatile memory port.
 8. A user equipment(UE) operable to perform device-to-device (D2D) discovery, the UEcomprising one or more processors configured to: identify a D2Ddiscovery resource that is M subframes in a time domain, wherein M is apositive integer greater than one; generate two demodulation referencesignal (DMRS) sequences to be transmitted from the UE for each subframein the D2D discovery resource; apply an orthogonal cover code (OCC) toeach DMRS sequence, wherein the OCC is randomly selected from a pool ofOCCs, wherein the OCCs in the pool are predefined based on a value of M;and transmit the two DMRS sequences for each of the M subframes of theD2D discovery resource.
 9. The UE of claim 8, wherein the one or moreprocessors are further configured to transmit the two DMRS sequences ina D2D discovery message using a physical uplink shared channel (PUSCH).10. The UE of claim 8, wherein the one or more processors are furtherconfigured to randomly select the D2D discovery resource from a D2Ddiscovery resource pool allocated by an evolved node B (eNB).
 11. The UEof claim 8, wherein the one or more processors are further configured totransmit the two DMRS sequences for each of the M subframes.
 12. Amethod for performing device-to-device (D2D) discovery, the methodcomprising: identifying, at a user equipment (UE), a D2D discoveryresource that is M subframes in a time domain, wherein M is a positiveinteger greater than one; generating K demodulation reference signal(DMRS) sequences to be transmitted from the UE for each subframe in theD2D discovery resource, wherein K is a positive integer greater thantwo; applying an orthogonal cover code (OCC) to each DMRS sequence,wherein the OCC is randomly selected from a pool of OCCs, wherein theOCCs in the pool are predefined based on a value of M and a value of K;and transmitting the K DMRS sequences for each of the M subframes of theD2D discovery resource from the UE.
 13. The method of claim 12, whereinthe K DMRS sequences for each of the M subframes is transmitted using aphysical uplink shared channel (PUSCH).
 14. The method of claim 12,wherein Type 1 D2D discovery or Type 2 D2D discovery is performed at theUE.
 15. A user equipment (UE) operable to perform device-to-device (D2D)discovery, the UE comprising: a selection module configured to: select afirst demodulation reference signal (DMRS) sequence from a pool of DMRSsequences for D2D discovery; and select a first D2D discovery resourcefrom a first D2D discovery resource pool allocated by an evolved node B,wherein the selection module is stored in a digital memory device or isimplemented in a hardware circuit; and a communication module configuredto transmit the first DMRS sequence from the UE using the first D2Ddiscovery resource selected from the D2D discovery resource pool,wherein a second DMRS sequence is subsequently transmitted from the UEusing a second D2D discovery resource that is selected from a second D2Ddiscovery resource pool allocated by the eNB, wherein the communicationmodule is stored in a digital memory device or is implemented in ahardware circuit.
 16. The UE of claim 15, wherein the selection moduleis further configured to: randomly select the first DMRS sequence fromthe pool of DMRS sequences; and select the second D2D discovery resourcebased on the first D2D discovery resource.
 17. The UE of claim 15,wherein the communication module is further configured to transmit aDMRS sequence in each discovery subzone within a configured discoveryperiod.
 18. The UE of claim 15, wherein the second DMRS sequence isidentical to the first DMRS sequence
 19. The UE of claim 15, furthercomprising a generation module configured to generate the second DMRSsequence by performing DMRS sequence hopping on the first DMRS sequence,wherein the DMRS sequence hopping utilizes at least one of: basesequence hopping, cyclic shift hopping, or orthogonal code coverhopping, wherein the generation module is stored in a digital memorydevice or is implemented in a hardware circuit.
 20. The UE of claim 15,wherein the selection module is further configured to randomly selectthe first D2D discovery resource from the D2D discovery resource pool,wherein the second D2D discovery resource is deterministicallyassociated with the first D2D discovery resource.
 21. The UE of claim15, wherein the selection module is further configured to randomlyselect the first DMRS sequence and the second DMRS sequence atsubstantially a same time, wherein the second DMRS sequence isdistinguishable from the first DMRS sequence.
 22. The UE of claim 15,wherein the selection module is further configured to randomly selectthe first D2D discovery resource and the second D2D discovery resourceat substantially a same time, wherein the second D2D discovery resourceis not associated with the first D2D discovery resource.
 23. The UE ofclaim 15, wherein the UE is configured to perform Type 1 D2D discoveryor Type 2 D2D discovery.
 24. The UE of claim 15, further comprising adiscovery module configured to perform the D2D discovery using a subsetof the pool of DMRS sequences, wherein each DMRS sequence in the subsetis associated with a configured number of cyclic shift (CS) and aconfigured number of orthogonal cover codes (OCCs), wherein thediscovery module is stored in a digital memory device or is implementedin a hardware circuit.